The present invention relates to a resin-molded semiconductor device in which a semiconductor chip and signal-connecting leads to be connected to the chip are encapsulated with a resin encapsulant, a method for manufacturing the device, a lead frame used for the device, and a method for manufacturing the lead frame.
In recent years, in order to catch up with rapidly advancing downsizing of an electronic appliance, it has become increasingly necessary to package semiconductor devices in the electronic appliance with higher and higher density. Such a semiconductor device is typically formed by encapsulating a semiconductor chip, including a plurality of semiconductor components, with a resin encapsulant. Thus, a semiconductor device of this type is called a "resin-molded semiconductor device". Accordingly, sizes and thicknesses of such resin-molded semiconductor devices have also been noticeably and drastically reduced to meet the demand for high-density packaging. Hereinafter, a conventional resin-molded semiconductor device will be described with reference to the accompanying drawings.
FIG. 16(a) is a plan view of a conventional resin-molded semiconductor device, in which only the contour of a resin encapsulant is illustrated by regarding the encapsulant as being transparent for convenience. FIG. 16(b) is a cross-sectional view of the device taken along the line XVIb--XVIb in FIG. 16(a). As shown in FIGS. 16(a) and 16(b), the conventional resin-molded semiconductor device is of the single-side encapsulated type where external terminals are arranged along the outer periphery on the lower surface thereof.
The conventional resin-molded semiconductor device uses a lead frame consisting of: inner leads 101; a die pad 102; and support leads 103 for supporting the die pad 102. A semiconductor chip 104 is bonded onto the die pad 102 of the lead frame with an adhesive, and electrode pads (not shown) of the chip 104 are electrically connected to the inner leads 101 with metal fine wires 105 used as connecting members. And part of the inner leads 101, the die pad 102, semiconductor chip 104, part of support leads 103 and metal fine wires 105 are encapsulated with a resin encapsulant 106.
In this resin-molded semiconductor device, no resin encapsulant 106 exists on the respective lower surfaces of the inner leads 101. In other words, the respective lower surfaces of the inner leads 101 are exposed and the respective lower parts of the inner leads 101, including the exposed lower surfaces thereof, are used as outer leads 107. To improve adhesion between the resin encapsulant 106 and the inner leads 101 or the die pad 102, side faces of the leads 101 and pad 102 are shaped like a taper with an upwardly increasing thickness, not to extend perpendicularly to the upper and lower surfaces of the frame.
In such a resin-molded semiconductor device, the respective lower surfaces of the resin encapsulant 106 and the die pad 102 are both located on the same plane. Stated otherwise, the lower surface of the lead frame is not substantially encapsulated. Accordingly, the thickness of such a semiconductor device is thinner than usual.
Hereinafter, it will be outlined how to manufacture the resin-molded semiconductor device shown in FIGS. 16(a) and 16(b).
First, a lead frame including inner leads 101 and a die pad 102 is prepared and then wrought mechanically or chemically to shape the side faces of respective parts of the lead frame like a taper.
Next, a semiconductor chip 104 is bonded onto the die pad 102 of the lead frame prepared, and is electrically connected to the inner leads 101 with metal fine wires 105. As the metal fine wires 105, aluminum (Al) or gold (Au) fine wires may be appropriately used, for example.
Then, the lead frame, on which the semiconductor chip 104 has been bonded, is introduced into a die assembly and transfer-molded. In particular, resin molding is performed with the lower surface of the lead frame in contact with an upper or lower die of the die assembly. That is to say, the die pad 102, semiconductor chip 104, inner leads 101, part of the support leads 103 and metal fine wires 105 are encapsulated within a resin encapsulant 106.
After the step of encapsulating has been performed, parts of outer leads 107, protruding outward from the resin encapsulant 106, are cut off to have a predetermined length, thereby completing a resin-molded semiconductor device.
In such a conventional resin-molded semiconductor device manufactured this way, the support leads might possibly be deformed during the manufacturing process step thereof. Then, the die pad, to be supported by the support leads, might be unintentionally out of balance and tilt or be displaced upward as a result. In such a case, the resin encapsulant would sometimes reach the lower surface of the die pad to form so-called "resin burr". If such resin burr exists on the lower surface of the die pad, then desired characteristics might not be attained in terms of heat radiation, for example. This is because the die pad could not be in satisfactory contact with a heat-radiating pad to be aligned with the die pad on a motherboard.
Such resin burr can be removed by using water jet or the like. However, such a deburring process is not just troublesome, but causes additional problems. Specifically, if a water jet process is carried out, then a nickel (Ni), palladium (Pd) or gold (Au) plated layer might peel off from the leads and impurity might deposit on the exposed parts thereof. Accordingly, such parts exposed on the resin encapsulant should be plated once again after the encapsulation. As a result, work efficiency and reliability of the device might possibly deteriorate.
In order to avoid such a problem, tilting of the die pad can be prevented by providing a bent portion, functioning as a spring, for each of the support leads to cushion the pressure causing the deformation of the support lead. However, if such a bent portion is provided for each support lead, then part of the support lead should be raised upward. Accordingly, a semiconductor chip cannot be mounted but inside a region defined by the respective raised portions of the support leads. That is to say, supposing the overall size of a resin-molded semiconductor device is constant, the size of an installable semiconductor chip is restricted if such bent portions are formed. Stated otherwise, the overall size of a resin-molded semiconductor device cannot be reduced below a certain limit. In addition, raising a part of each support lead upward imposes unwanted restriction on the reduction in thickness of a resin-molded semiconductor device using such support leads.
Also, the conventional resin-molded semiconductor device has a structure where the resin encapsulant is adhered to substantially one face only of the lead frame having the semiconductor chip mounted thereon, i.e., the upper face of the lead frame, to reduce the resulting thickness. In other words, since the contact area between the lead frame and the resin encapsulant is smaller than usual, the adhesion therebetween is less than satisfactory. Therefore, water or moisture is more likely to penetrate between the die pad and the resin encapsulant, resulting in even poorer adhesion between these members or forming cracks in the encapsulant. What is worse, since the die pad cannot adhere to the encapsulant, either, the moisture resistance of the device might possibly deteriorate.